The invention relates to a grid for holding fuel rods in a nuclear fuel assembly, and in particular to those used in pressurized water reactors to hold the rods at the nodes of a regular array, generally a square array.
The invention relates in particular to the structure of the straps which are assembled together to make up the grids, generally by means of half-depth slots.
The grids of an assembly define common or regular cells designed to receive rods, and other cells which have guide tubes passing through them, the guide tubes often being welded to the grids. Conventionally, in the grids used for holding rods axially and laterally, rigid bearing dimples are formed for the rods in two of the four faces of each common cell for the purpose of receiving a rod, and two holding springs are cut out from or added to the other two faces so as to press the rod onto the dimples.
The invention relates in particular to the straps of a grid in which the springs are integrally formed with the flat portion of each strap. At least in the portion of the assembly which is subjected to a high level of neutron flux, these straps are made of a zirconium based alloy such as Zircaloy 4. Under neutron radiation, the mechanical characteristics of such alloys are degraded. Proposals have already been made to limit the consequences of such degradation by making the springs in special manner. Document FR-A-2 338 549 describes a grid strap in which each spring comprises a flexible corrugated strip extending in the flow direction of the coolant, i.e. transversely relative to the strap, or two parallel strips that are interconnected in their middle portions by a portion which projects into the flow and which is extended, between the strips, by two tabs. This projecting portion constitutes an obstacle which disturbs the flow of coolant. Document WO-A-94/25935 describes a spring constituted by a resilient strip whose ends are connected to the current portion of the strap via two stamped bridges. Given their orientation, the bridges do not hinder the flow of coolant to any significant extent, but the bend taken up by the blade when pressed against a rod still constitutes an obstacle to the flow.
It is an object of the invention to provide a strap having holding springs of a structure such that they provide a satisfactory compromise between mechanical strength, conserving a satisfactory level of resilient holding force, and high transparency to the flow of coolant.
To this end, the invention provides, in particular, a grid strap for holding fuel rods, in which there are formed notches at regular intervals for assembly with transverse straps, and between at least some pairs of notches, rod-holding springs each comprising two resilient strips cut out from the plane wall of the strap and integral therewith, each strip extending in the axial flow direction in a grid including the strap and is connected to the strap at its two ends, the strips projecting, at least when in released condition, towards the inside of the cell concerned and being interconnected in their middles by a transverse bridge projecting relative to both strips.
The springs of an internal strap may all have the same structure, or they may be of different structures, for example depending on whether the wall in which the spring is formed separates two cells that are both occupied by a rod, or separates two cells, one of which is occupied by a guide tube.
In the first case, the two resilient strips advantageously join together in the vicinity of their roots, i.e. their zones close to where they connect to the current portion of the strap, and are on the contrary remote from each other in their central portions which are interconnected by the bridge. In particular, the portions close to the roots and the central portions may be rectilinear in the flow direction; each root is then connected to the corresponding central portion via a zone that is curved or oblique. Such a structure avoids having large openings in the strap which could disturb the flow, in particular by allowing uncontrolled interchanges between cells.
In the second case, the strap constitutes a cell wall backing a guide tube; the spring can be constituted by two mutually parallel strips in the flow direction, having their central portions interconnected by the bridge. There is no drawback in having an opening in the strap at this location, given that the opening is closed by the guide tube.
On either side of such a spring, in the flow direction, there remains a substantially flat zone in which the strap can be fixed to the guide tube by welding.
The rod is provided with support on its side opposite from its side against which the spring presses. For this reason, the spring, situated in a middle portion of the strap, is generally located between two bearing areas constituted by stamped bridges extending transversely to the flow so as to avoid significantly altering the flow section for the coolant. The bridges of the springs and those of the bearing areas may be of the same shape.
The initial shape given to the spring strips by stamping is advantageously such that the strips are pushed back substantially into the same plane as the current portion of the strap when a rod is inserted and is to be held by the spring. The bridges and the dimples advantageously project equally into the cell when the springs are in the active position.
The straps may also be provided with deflector fins for improving mixing of the coolant, which fins are curved in shape.
As a general rule, the grid has outer straps constituting a belt and in abutment against innner straps via tenon-and-mortise type links. The outer straps may have guide fins for limiting the risk of catching between the grids of adjacent fuel assemblies while a fuel assembly is being loaded or unloaded.
In an advantageous embodiment, the outside plates are curved at the base of the guide fins so as to follow the shape of the terminal edges of the common inner straps which have curved shapes for this purpose.
The outer straps may also contribute to holding peripheral rods and may be provided with springs. Given the relatively small number of grid cells concerned, and in order to make these straps very strong, the springs which are obtained by cutting out and stamping, may be disposed in the flow direction.
In particular, the straps may be made out of one of the zirconium alloys known as xe2x80x9cZircaloy 4xe2x80x9d, in the recrystallized state.
The above characteristics, advantageously usable in combination, but capable of being used separately, together with other characteristics, appear more clearly on reading the following description of particular embodiments, given as non-limiting examples. The description refers to the accompanying drawings.